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SINHA

AND

K. P. SINGH

The s o l u b i l i t y of h y d r o g e n was d e t e r m i n e d in the ( Z r + 5 wt pct Nb)-H2, (Zr + 10 wt pct Nb)He, and (Zr + 20 wt pct Nb)-H2 s y s t e m s a s a function of c o m p o s i t i o n , t e m p e r a t u r e (700 ~ to 950~ a n d hydrogen e q u i l i b r i u m p r e s s u r e (0.5 to 760 m m Hg). The p o s i t i o n of b o u n d a r i e s ~-([3 + 5) and (/3 + 5)-5 w e r e d e t e r m i n e d in each of the above t h r e e s y s t e m s . N i o b i u m s i g n i f i c a n t l y r e d u c e s the s o l u b i l i t y of hydrogen in the /3 and 5 p h a s e s and i n c r e a s e s the e q u i l i b r i u m h y d r o g e n p r e s s u r e for any fixed c o n c e n t r a t i o n . The e q u i l i b r i u m p r e s s u r e - t e m p e r a t u r e r e l a t i o n s in the two p h a s e region (/3 + 5) w e r e d e r i v e d and the heat of f o r m a t i o n of 5h y d r i d e f r o m s a t u r a t e d f~-Zr, AH~ ~ 5, w e r e d e t e r m i n e d . The value of --AH/~ ~ 6 i n c r e a s e s up to about 5 wt pct Nb a f t e r which the effect of n i o b i u m s e e m s to be i n s i g n i f i c a n t . The m a x i m u m hydrogen p i c k - u p of z i r c o n i u m at r o o m t e m p e r a t u r e d e c r e a s e s with i n c r e a s i n g n i o b i u m content of the a l l o y .

BY v i r t u e

of t h e i r p h y s i c o c h e m i c a l and t e c h n o l o g i c a l p r o p e r t i e s , the Z r - N b a l l o y s a r e of potential i n t e r e s t for f u e l - e l e m e n t cladding and p r e s s u r e t u b e s in w a t e r cooled n u c l e a r power r e a c t o r s . However, d u r i n g r e a c t o r o p e r a t i o n , the h y d r i d e p r e c i p i t a t i o n in the alloy, which l i m i t s the life of the f u e l - e l e m e n t cladding and p r e s s u r e tube m a t e r i a l , is i n e v i t a b l e . T h e r e f o r e to s i m u l a t e the conditions in the r e a c t o r and to u n d e r stand the effect of n i o b i u m on the h y d r i d e p r e c i p i t a t i o n , the study of hydrogen s o l u b i l i t y in Z r - N b a l l o y s b e comes important. E x t e n s i v e t h e o r e t i c a l 1-3 and e x p e r i m e n t a l 4-17 s t u d i e s of the Z r - H s y s t e m have m o r e o r l e s s c o m p l e t e l y e s t a b l i s h e d the b i n a r y p h a s e d i a g r a m , c o n f i r m i n g the e x i s t e n c e of t h r e e h y d r i d e p h a s e s , 5 (fcc), 7 (fct), and r (fct) in addition to the ~ and fl p h a s e s , which a r e the low and high t e m p e r a t u r e m o d i f i c a t i o n s of p u r e z i r c o n i u m . The r e s u l t s of s e v e r a l i n v e s t i g a t o r s have b e e n r e c e n t l y s u m m a r i z e d by Beck and Mueller.~S However, t h e r e is no p u b l i s h e d i n f o r m a t i o n on the Z r Nb-H s y s t e m , except the s o l u b i l i t y of hydrogen in Z r 2.5 wt pet Nb alloy by Sinha and Singh, ~9 E r i c k s o n , 2~ Slattery, 21 Sawatzky and W i l k i n s , z2 K e a r n s , 2s and the i n f l u e n c e of a l l o y i n g